Drilling protection bolts for climbing is permitted in Yosemite as long as it is done by hand. Motorized power drills are prohibited. The National Park Service does not inspect, maintain, or repair bolts and other climbing equipment anywhere in the park.
Beyond this simple rule, there is a strong community bolting ethic in Yosemite. If you plan to bolt a new route or alter an existing one, talk with local climbers who are familiar with Yosemite's route history and traditions before permanently altering the cliff face. No one wants to see the rock damaged by bolts being placed and chopped.
"Gardening" (the name given to removing plant life from cracks) is not allowed in Yosemite. Many climbers remove the occasional bit of grass or leaves to place protection or find a finger-lock, but this is nothing compared to the serious damage done establishing a new area.
The damage caused establishing a new route is far greater than that caused by each subsequent party. If you are considering establishing a new route ask yourself, "Is this route worth the damage it will cause?" "Is it a classic line that others will enjoy climbing, or I am simply interested in putting up my own route?" "What will climbers fifty years from now think of this route or this bolt?" There are thousands of established routes in Yosemite already--maybe try a few more of those before making a new mark on Yosemite's Wilderness.
Most of the Yosemite's climbing areas are in designated Wilderness, and motorized items, including power drills, are not allowed in these areas. In addition to this Congressional mandate, the park has an interest in limiting the impacts from climbing while enabling climbers to enjoy the park. The resulting rule allows climbers the unusual privilege of permanently altering Yosemite's granite cliffs by adding bolts in the location of their choosing, but inherently limits the number of those bolts by requiring that they be hand drilled.